The problem in this case has nothing to do with mold temperature, or fridge temperature, or even humidity. It has to do with the type of crystal the cocoa butter has formed in the tempering cycle.
I've seen this happen many times when I'm teaching a new employee to temper chocolate. They go through the tempering cycle too quickly, and then bar up the chocolate. The bars at first come out of the cooler with a super nice shine, but within 24 hours, the inside of the bar turns all grainy, and eventually all look exactly like the photo.
Solving this problem is very simple. When taking the chocolate through it's tempering cycle either:
- Lower the cooling temperature a couple of degrees (allowing more crystal propogation)or
- Let the chocolate remain at the cooling temperature for a longer period of time.
The problem you are having is that you are not allowing enough time for the appropriate types of crystals to propogate, BEFORE you reheat yoru chocolate to the working temperature to mold up your bars.
You also need to remember that when working with different kinds of beans and making single source varieties of chocolate, the cocoa butter in each will exhibit different behaviours, and will temper slightly differently from the others. The tempering cycle is more of a "rule of thumb" for tempering all chocolate, but with experience, you will see that each chocolate you make will temper slightly differently. It's not something you can just apply a boiler plate heat/cool/reheat cycle to (although for small batches you will get VERY close).
I hope this helps.
Cheers and Happy Chocolate Making!